Engines operating with a variable number of active or deactivated cylinders may be used to increase fuel economy, while optionally maintaining the overall exhaust mixture air-fuel ratio about stoichiometry. In some examples, half of an engine's cylinders may be disabled during selected conditions, where the selected conditions can be defined by parameters such as a speed/load window, as well as various other operating conditions including vehicle speed. A VDE (Variable Displacement Engine) control system may disable selected cylinders through the control of a plurality of cylinder valve deactivators that affect the operation of the cylinder's intake and exhaust valves. Variable displacement capabilities can be combined with, for example, variable cam timing (VCT), to further improve fuel economy and emissions performance of the vehicle.
However, as recognized by the inventors herein, a potential issue with variable displacement engines may occur when transitioning operation between the various displacement modes, for example, when transitioning from a non-VDE (or full-cylinder) mode to a VDE (or reduced cylinder) mode, and vice-versa. Specifically, the transitions can interact with the motive force used to move the VCT mechanisms. Consequently, a command issued by an engine controller can cause substantially different responses in the VDE mechanism and the VCT mechanism. For example, when a constant cam position is commanded, the position of the VCT phaser may not be suitably maintained during the various VDE valve transitions without adversely affecting gain stability limits.
Thus, the inventors herein have recognized that for an engine equipped with both VCT and variable displacement capabilities, the systems require coordination. Thus, in one example, some of the issues may be addressed by a method of controlling an engine, the engine including a first group of cylinders having selectively deactivatable cylinders valves, the engine further including a variable camshaft timing actuator coupled in a hydraulic system, the method comprising: during a first mode of engine operation where the first group of cylinders is activated to carry out combustion cycles, adjusting the hydraulic system with a first gain based on cam timing feedback information; and during a second mode of engine operation where the first group of cylinders is deactivated, adjusting the hydraulic system with a second gain based on cam timing feedback information, the second gain higher than the first gain. Further, in another example, the method may include during the first mode of engine operation maintaining a null position of a hydraulic actuator by adjusting a control signal to the hydraulic system to within a first value range; and during the second mode of engine operation, maintaining the null position of the hydraulic actuator by adjusting the signal to within a second value range.
In this way, different feedback control gains may be used to control actuation of the variable cam timing while taking into account response effects caused by operation in different cylinder deactivation/activation modes. Further, different null holding signals may also be used for the variable cam timing system that take into account effects caused by operation in different cylinder deactivation/activation modes
In one particular example, a variable displacement engine may be configured to operate with cam-torque actuated variable cam timing. A feedback control loop may be configured to convert position errors in VCT positioning into corresponding integral and proportional terms via control gains. The gain characteristics as well as the feed-forward response characteristics may be adjusted based on the status of the selectively deactivatable cylinder valves, for example, whether the engine is in a VDE or non-VDE mode of operation. Based on the output of the feedback control loop, VCT phaser settings may be appropriately adjusted. For example, the phasing rate, the duty cycle, and/or the null position of the VCT phaser may be appropriately adjusted responsive to valve activation/deactivation conditions. In one example, the duty cycle command may be adjusted more aggressively when the engine is in a VDE mode of operation, to compensate for the slower movement of the cams. In another example, the null position may be adjusted to a lower duty cycle when the engine is in a non-VDE mode of operation. By adjusting VCT phaser commands responsive to the VDE mode of the engine, disturbances may be better rejected and controller stability may be further increased, particularly during VDE/non-VDE transitions.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.